The proliferation of microcontrollers in automobile systems is a major advance in the art, one making it possible to control many functions more economically and precisely as compared to simple switches used in the past. The microcontroller is a critical element in a vehicle "electronic module", which, in most cases, provides an array of outputs capable of providing control signals for equipment and systems such as wipers, power windows and headlamps. The microcontroller has analog and digital inputs to receive signals from analog and digital input devices, such as switches, potentiometers and digital sensors and signal processors. However, a typical low cost controller, such as the Motorolla model MC 68HC05B6, has a limited number of analog and digital inputs, placing limits on the number of input channels to an electronic module using it. This places practical limits on the complexity and number of devices and functions that the microcontroller can control.
In automobile applications, a rudimentary way to provide a suitable digital input to a microcontroller is to apply battery voltage from a switch through a resistor to a controller input having a zener diode clamp to clamp the in/out voltage. The series resistor limits current to the microcontroller input; the zener diode limits input voltage to a safe input TTL logic level, usually in the range of 5 volts peak. These traditional methods can be very costly, however, when a significant number of analog and digital inputs to the microcontroller are used.